The Mechanical Behavior Of Asphaltoverlays Over The Rubblized Pcc Slabs

In recent years, with the rapid growing of the demands of the rehabilitation and asphalt overlays over PCC pavements, it turns out to be urgent for how to solve the key technical problems and engineering issues, improve the functional pavement performance and ensure the quality of driving when considering save the investment. However, the key technology of the reconstruction of the old cement concrete pavement is the treatment method of the damaged concrete slab. Rubblization of PCC pavement is an optional technique to rubblize the slab into fragments, which includes transforming the original concrete slabs into an aggregate base or subbase. It's better to use the structural capacity of the existing pavements, which not only reduces the investment and effectively eliminates the reflective cracking, but also solves the waste disposal problems. Although the rubblization has already been applied in the domestic engineering, it's still need to refine and has not yet reached a consensus on the design method of asphalt overlays over PCC slabs under heavy traffic load. So there's much research work need to be done, such as the mechanical mechanism of rubblizing PCC pavement, the design method of AC overlays, the properties and pavement performances of the rubblized layer, etc. In this work, through theoretical research, numerical analysis, indoor and outdoor tests and other means, the mechanism behavior of the asphalt overlay over the rubblized PCC slabs is discussed compound with the extensive research and Chengdu-Mianyang highway reconstruction projects.At first, this paper carries out reseach on the properties of rubblized layer by the means of field investigating, core sampling and test pits excavating. It obtains the basic physical parameters and the gradation of rubblized layer. The resilient modulus of pavement materials has been backcalculated through the FWD testing data of experimental roads,according to the deflection equivalence principle.The surface deflection is actually a comprehensive response to the load, and the maximum proportion of the vertical deformation is caused by the subgrade. So, according to the load diffusion law, the backcalculated process of modulus is that the modulus of subgrade is first back calculated on basis of the deflection, which is away from the bearing plate. Then the modulus of the other structure layer of pavement is back calculated on the basis of the deflection near the bearing plate. Because it's the difference the mechanical response under the FWD dynamic loading from that under the static load, the measured FWD data that pre and post rubblization is used to back calculate the parameter of pavement materials using the finite-element software of ABAQUS, which is applied to secondary developed of using the python computer language, combined with Chendu-Mianyang highway reconstruction projects.The results suggest this program can give rational mechanical parameters of pavement structure.Then, combined with the actual conditions on site, two typical structures of asphalt overlays over the rubblized PCC slabs, which the broken particle size of the structure 1 is less than 15 cm, while the broken particle size of the structure 2 is less than 7.5 cm, is selected to carry out the full-scale fatigue experiment. The changes of the mechanical condition of the asphalt overlays and rubblized layer is studied, caused by the cumulative effect of the vertical dynamic loads. Meanwhile, the deterioration relationship between the physical indicators of the rubblied layer and overlays is discussed under the control of the environment and load conditions. The results show that the strain response of pavement varied with the load conditions.And the strain with load repetition has shown an increasing trend. The strain rate at the bottom of asphalt overlays varies with the particle size of the rubblized layer under the same load.The strain rate of the pavement structure 1 is 0.9119,which is larger than 0.3564 of the pavement structure 2. It indicated that the particle size has great influenced on the fatigue property of the alphalt overlays.To understand how the characteristics of mechanical behavior is affected by the degree of rubblization slabs, a numerical method which combines the finite difference and DEM is proposed, in which the rubblized PCC slab is described using DEM while the asphalt overlays, base and subgrade is simulated by the finite difference method, according to the actual structure. The conditions of mandatory continuity along the corresponding boundaries need to be satisfied in the computation process. To illustrate the effects of rubblization on asphalt overlays pavements more clearly, rubblized PCC slabs is further divided into three sublayers as the small particles layer, the surface layer and bottom layer. The calculation methods of material combination of different scales are discussed. The results show that the thicker the small particles layer is, the higher the degree of rubblization of the slabs is and the larger the tensile stress, strain and deflection at the bottom of the asphalt overlays are. It also found that the particle size of a small particles layers is increased about 1mm to 2mm, the tensile stress and strain at the bottom of the asphalt overlays are proportional to the particle size of the small particles layer; while the particle size is reduced from 5-8mm to 3-5mm, the smaller the particles size of the surface layer is, the higher the degree of rubblization of the slabs is and the larger the tensile stress and strain are. In other words, strength loss of the old PCC is proportional to degree rubblization of the slab. This shows that the degree of rubblization should be properly controlled in the reconstruction projects.Finally, combined with the actual structure of asphalt overlays over the rubblized PCC slabs, the significant factors affect the reliability of the asphalt overlays over the rubblized PCC slabs are determined. The two state equations are established in which the design deflection and the allowable tensile stress at the bottom of the asphalt layer are determined as the main index respectively. And the program of the reliability and the sensitiving analysis of the pavement structure is the secondary developed in the 2D finite element computer program platform.The results show that the factors which affect the reliability of the structure of pavement are as fellows:subgrade, the rubblized layer, base and asphalt overlays,according to the allowable tensile stress at the bottom of the asphalt layer. Among them, the factors of the subgrade and the rubblized layer are more than 85%,which are the decisive factor of the resistance to tensile strength of the structure.While the factors which affect the reliability of the structure of pavement are as fellows:asphalt overlays, base, subgrade and the rubblized layer, according to the surface deflection; Among them, the factors of the asphalt overlays and baser are more than 90%,which is the decisive factor of the resistance to deformation of the surface deflection. Therefore, seeking a balance in order to achieve overall optimum to the performance of the pavement structure under the load, two main factors which affect the reliability of the structure of pavement should be taken into account,comprehensively,in the design and construction.